Summary of Work: The goals of this project are to understand the biochemistry and genetics of MMR in normal eukaryotic cells, and how mutations in MMR genes lead to environmentally associated human diseases. This year we completed studies showing that the Mlh1-Pms1 heterodimer contains two separate DNA binding sites and provided evidence suggesting that DNA binding by Mlh1 is important for MMR function. We completed collaborative studies showing that cadmium inhibits MMR activity in yeast and in human cell free extracts. We established the first knock-in mouse model harboring a missense mutation in the mismatch repair gene Msh2 that is homologous to a mutation associated with early onset ovarian cancer in humans. This mutation results in delayed onset cancer susceptibility and it inactivates MMR but does not inactivate apoptosis induced by the chemotherapeutic drug cisplatin. This study reveals that cancer susceptibility resulting from defects in mismatch repair genes reflects the roles of mismatch repair proteins both in repairing replication errors and in determining cellular toxicity following environmental DNA damage. Background: Rare DNA synthesis errors are corrected by post-replication DNA mismatch repair (MMR). Loss of MMR increases mutation rates and decreases apoptosis in response to certain forms of DNA damage, ultimately leading to cancer. MMR proteins also prevent recombination between partially homologous DNA and participate in other types of repair of DNA damage. They also function in meiotic recombination, and mutations in certain MMR genes result in infertility. The goals of this project are to understand the biochemistry and genetics of MMR protein function in normal eukaryotic cells, and how mutations in MMR genes lead to environmentally associated diseases.